Recently enacted United States federal law requires that municipal sludges be pasteurized to remove harmful pathogens. The sludge must be passed through a non-agitation type retention tank and held at a temperature of at least 158.degree. F. for at least 30 minutes. Heating large quantities of sludge to 158.degree. F. for a period of 30 minutes presents a difficult and challenging engineering problem.
Submerged combustion heating is a method whereby hot products of combustion are forced through a liquid or liquid-solid mixture to heat the liquid or liquid-solid mixture. A major advantage of this heating system is that the heat exchange occurs directly between the hot gaseous products of combustion and the liquid or liquid-solid. Thus there is no interface that interferes with heat exchange. In a submerged combustion system, the hot combustion products are generated by a flame which is typically fuelled by a combination of air and natural gas. The flame generates hot combustion gases which contact the liquid or liquid-solid to be heated, but the flame itself does not come into contact with the liquid or liquid-solid.
This submerged combustion technology differs from conventional heat exchange methods such as immersion tube heating where the heat exchange is indirect through a solid interface and the products of combustion are exhausted directly to the atmosphere, rather than being forced through the liquid. Submerged combustion can be utilized to heat liquids with overall system efficiency greater than 90%. Conventional hot water boiler indirect heating systems have an efficiency of about 80%. Immersion tube heating systems are relatively low performers and have an efficiency of about 70%.
In applications where separation of components by distillation or absorption is required, submerged combustion heating can be applied to generate liquid or liquid-solid temperatures up to about 195.degree. F. This is not much below the boiling point of water, and is applicable to most industrial and domestic liquid or liquid-solids heating applications.
In addition to high efficiency, submerged combustion heating systems are advantageous because they maintain a uniform temperature throughout the liquid or liquid-solid in which the submerged combustion is conducted. This is because the hot gaseous combustion products pass rapidly through the liquid and keep the liquid in constant agitation, thereby distributing heat evenly. Submerged combustion heating systems are also suitable for heating contaminated liquids, or liquids with low medium or high solids contents. Expenses are usually lower than with other heating systems because the submerged combustion heating can be conducted in a liquid holding tank which can operate at ambient pressures, thereby eliminating the need to be pressurized. Unlike boiler heating applications, a certified operating engineer is not required to operate a submerged combustion heating system.
A typical industrial application for a submerged combustion system is a municipal effluent holding and treatment pond, which can include maintenance of pond temperatures to ensure continuous high level of biological degradation especially in regions that experience extreme seasonal temperature changes, and in other cases, elevated temperatures to pasteurize the effluent.
The applicant is the assignee of one or more of the inventors herein and therefore the owner of the following patents relating to a submerged combustion heating system:
1. U.S. Pat. No. 5,606,965, granted Mar. 4, 1997 entitled "Submerged Combustion System"; PA1 2. U.S. Pat. No. 5,615,668, granted Apr. 1, 1997, entitled "Apparatus for Cooling Combustion Chamber in a Submerged Combustion Heating System"; and PA1 3. U.S. Pat. No. 5,636,623, granted Jun. 10, 1997, entitled "Method and Apparatus for Minimizing Turbulence in a Submerged Combustion System". PA1 4. U.S. Pat. No. 5,032,230, Sheppard, discloses a system for evaporating large quantities of liquid using a vacuum system.
The subject matter and contents of the first three aforementioned U.S. patents is incorporated herein by reference.